Progressive multifocal lenses are spectacle lenses which assist a person's vision when the eye's accommodation ability weakens and seeing nearby objects becomes difficult. In general, progressive multifocal lenses are provided with a distance vision correction zone (hereinafter "distance part") located in the upper portion of the spectacle lens when being worn; a near vision correction zone (hereinafter "near part") in the lower portion; and a progressive zone (hereinafter "intermediate part") between these two zones where the refractive power changes progressively or continuously. Furthermore, in the apparatus incorporating the principles of the present invention, "upper part," "lower part," "horizontal," "vertical," etc. indicate positional relationships when the spectacle lenses are being worn by an individual person. The difference between the refractive power for near vision and the distance vision is called the addition.
In general, if a wide clear vision zone (astigmatic difference in the range of 0.5 diopters or less) is ensured in the distance part and in the near part of a progressive multifocal lens, and they are linked by a progressive zone (progressive corridor), lens aberration is concentrated in the side zones of this progressive band. As a result, imaging failure (blurred image) and image distortion occur in the side zones of the progressive corridor. Also, when the line of sight moves to a side zone, the eyeglass wearer becomes aware of image distortion and image swim and image blurring which leads to an uncomfortable and negative feeling when wearing the eyeglasses.
In order to resolve these vision characteristics problems, known progressive multifocal lenses have been designed and evaluated based on various perspectives. For the shape of the lens surface, the intersection (main meridian) of a cross-section along a meridian running vertically through substantially the center of the lens surface from the upper part to the lower part with the lens surface object side is used as a reference line for establishing the specifications of the lens addition, etc., and is also used as an important reference line in lens design.
Progressive multifocal lenses with the near part asymmetrically arranged (hereinafter "asymmetric progressive multifocal lenses") have been proposed. Their near part is slightly shifted from the center toward the nose side when a lens is being worn.
The centerline consisting of the intersection of a cross-section running through the distance center and the near center with the lens surface object side is also used as a reference line in this sort of asymmetric progressive multifocal lens. In the apparatus incorporating the principles of the present invention, this reference line is generally referred to as the "main meridian curve."
The progressive multifocal lens disclosed in Japanese Laid-Open Patent Application S62-10617, which is for both intermediate and nearby use, is central to the type of technical background described above. This progressive multifocal lens for both intermediate and near use is a progressive multifocal lens based on a design which emphasizes intermediate vision to nearby vision. Compared to progressive multifocal lenses for both distance and near vision, image swimming and distortion are minimal, and the field of vision is relatively wide from a near distance to intermediate distances. It is said that such an arrangement provides eyeglasses that are relatively easy to use, particularly indoors.
However, as the eye's accommodation ability becomes weaker it is necessary to wear lenses with larger addition. In general, the larger the addition the more noticeable the defects of the progressive multifocal lens described above. That is to say, the larger the addition, the narrower the clear vision zone at the distance part and at the near part. As a result, it is not possible to move the line of sight and have comfortable side vision in the distance part and the near part. Accordingly, it is necessary to swing the entire head to provide proper side vision. The larger the addition, the greater the lens aberration in the side zone of the progressive band linking the distance part with the near part. As a result, when the line of vision moves to the side zone of the progressive corridor, image swimming, distortion and image blurring increase. In such case the sensation of wearing eyeglasses becomes very troublesome.
Also, conventional progressive multifocal lenses are designed to permit an eyeglass wearer to try to see well from a long distance as well as a short distance regardless of the extent of weakening of the eye's accommodation ability. Thus, the progressive band is relatively long. Therefore, when a lens is inserted into the eyeglass frame, the near vision zone is positioned at the lowest portion of the frame, and the line of sight must be greatly lowered when looking at a short distance (for reading, for example). As a result, seeing is difficult and moreover the greater lowering of the line of sight causes eyestrain. Therefore it is hard to use conventional progressive multifocal lenses continuously for relatively long periods of time when doing close-up activities such as deskwork, for example.
However, the conventional progressive multifocal lens for both intermediate and near use disclosed in Japanese Laid-Open Patent Application S62-10617 has a relatively long progressive corridor so to some extent it solves defects such as image swimming and distortion which are seen in ordinary progressive multifocal lenses. Nevertheless, as described above, the progressive band is long, so to see an object at a short distance, the line of sight must be greatly lowered. Accordingly, such an arrangement has the problem that seeing at short distances is difficult and causes eyestrain.